This application is based on Application No. 2001-7924, filed in Japan on Jan. 16, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an automotive alternator and particularly to an automotive alternator in which cooling of a stator is improved by making cooling airflows flow through cooling airflow ventilation channels formed by coil end groups of a stator winding and tooth portions of a stator core.
2. Description of the Related Art
FIG. 22 is a cross section showing a conventional automotive alternator, FIG. 23 is a perspective showing a stator used in the conventional automotive alternator, FIG. 24 is a schematic diagram explaining a method for manufacturing a conventional stator core, and FIG. 25 is a plan showing the conventional stator core.
In FIGS. 22 and 23, the conventional automotive alternator includes: a case 3 constituted by an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 6 disposed inside the case 3, a pulley 4 being secured to a first end of the shaft 6; a Lundell-type rotor 7 secured to the shaft 6; cooling fans 5 secured to first and second axial end portions of the rotor 7; a stator 8 secured to the case 3 so as to envelop the rotor 7; slip rings 9 secured to a second end of the shaft 6 for supplying electric current to the rotor 7; a pair of brushes 10 sliding on surfaces of the slip rings 9; a brush holder 11 accommodating the brushes 10; a rectifier 12 having a rectifier heat sink 12a, the rectifier 12 being electrically connected to the stator 8 for converting alternating current generated in the stator 8 into direct current; and a regulator 18 mounted to a regulator heat sink 17 fitted onto the brush holder 11, the regulator 18 adjusting the magnitude of the alternating voltage generated in the stator 8.
The rotor 7 is constituted by: a field winding 13 for generating magnetic flux on passage of an electric current; and a pair of first and second pole cores 20 and 21 disposed so as to cover the field winding 13, magnetic poles being formed in the first and second pole cores 20 and 21 by magnetic flux generated in the field winding 13. The pair of first and second pole cores 20 and 21 are made of iron, each has a plurality of first and second claw-shaped magnetic poles 22 and 23 having a generally trapezoidal outermost diameter surface shape disposed on an outer circumferential edge portion at even angular pitch in a circumferential direction so as to project axially, and the first and second pole cores 20 and 21 are fixed to the shaft 6 facing each other such that the first and second claw-shaped magnetic poles 22 and 23 intermesh.
The stator 8 is constituted by: a cylindrical stator core 15 in which slots 33 extending parallel to an axial direction are arranged at an even angular pitch in a circumferential direction; and a stator winding 16 installed in the slots 33 of the stator core 15. The stator winding 16 is constituted by three wave-winding phase portions each formed by installing a conductor wire 29, functioning as an electrical conductor composed of a copper wire material having a circular cross section coated with electrical insulation, into a wave shape in every third slot 33. The wave-winding phase portions are each installed in the stator core 15 such that the slots 33 in which each wave-winding phase portion is installed are offset by one slot from those of each of the other wave-winding phase portions. In addition, the wave-winding phase portions are each formed by winding the conductor wire 29 into a distributed winding. The stator 8 is held between the front bracket 1 and the rear bracket 2 so as to form a uniform air gap between outer circumferential surfaces of the first and second claw-shaped magnetic poles 22 and 23 and an inner circumferential surface of the stator core 15.
Moreover, the number of magnetic poles in the rotor 7 is twelve, and there are thirty-six slots 33 formed in the stator core 15. In other words, the slots are formed at a ratio of one per phase per pole. The stator winding 16 is formed into a three-phase alternating-current winding by forming the three wave-winding phase portions into an alternating-current connection (a Y connection, for example).
A method for manufacturing the stator core 15 will now be explained with reference to FIG. 24.
First, a long magnetic steel plate 30 is supplied to a pressworking machine (not shown), and formed into tooth portions 30a and a base portion 30b. Then, the magnetic steel plate 30 is supplied to a core manufacturing device (not shown). Here, the magnetic steel plate 30 is bent and wound up into a helical shape by intermeshing pins 34 in gaps 30c defined by the tooth portions 30a, and the base portion 30b. as shown in FIG. 24. The magnetic steel plate 30 is laminated to a predetermined thickness and then cut. Outer circumferential portions of the magnetic steel plate wound up in this manner are welded to obtain the stator core 15 shown in FIG. 25. Here, the tooth portions 30a and the base portion 30b are each stacked up in the wound magnetic steel plate 30 in the direction of lamination.
As shown in FIG. 25, the stator core 15 manufactured in this manner includes: a cylindrical base portion 32; tooth portions 31 each extending from an inner circumferential surface of the base portion 32 toward an axial center; and the slots 33, which are defined by the base portion 32 and adjacent pairs of the tooth portions 31. The tooth portions 31 are disposed at an even angular pitch on the inner circumferential surface of the base portion 32.
In the conventional automotive alternator constructed in this manner, an electric current is supplied from a battery (not shown) through the brushes 10 and the slip rings 9 to the field winding 13, generating a magnetic flux. The first claw-shaped magnetic poles 22 on the first pole core 20 are magnetized into North-seeking (N) poles by this magnetic flux, and the second claw-shaped magnetic poles 23 on the second pole core 21 are magnetized into South-seeking (S) poles.
At the same time, the pulley 4 is driven by an engine and the rotor 7 is rotated by the shaft 6. A rotating magnetic field is applied to the stator core 15 due to the rotation of the rotor 7, generating an electromotive force in the stator winding 16. Then, the alternating electromotive force generated in the stator winding 16 is converted into direct current by the rectifier 12 and the magnitude of the output voltage thereof is adjusted by the regulator 18, recharging the battery.
Now, the field winding 13, the stator winding 16, the rectifier 12, and the regulator 18 are constantly generating heat during power generation, and in an automotive alternator having a rated output current in the 100 A class, the amount of heat generated at rotational frequencies at which the temperature is high is 60 W, 500 W, 120 W, and 6 W, respectively.
Thus, in order to cool the heat generated by power generation, front-end and rear-end air intake apertures 1a and 2a are disposed through axial end surfaces of the front bracket 1 and the rear bracket 2, and front-end and rear-end air discharge apertures 1b and 2b are disposed through radial side surfaces of the front bracket 1 and the rear bracket 2 so as to face coil end groups 16f and 16r of the stator winding 16.
Thus, the cooling fans 5 are rotated and driven together with the rotation of the rotor 7, and cooling airflow channels are formed in which external air is sucked inside the case 3 through the front-end and rear-end air intake apertures 1a and 2a, flows axially towards the rotor 7, is then deflected centrifugally by the cooling fans 5, thereafter crosses the coil end groups 16f and 16r, and is discharged outside through the front-end and rear-end air discharge apertures 1b and 2b. Furthermore, as a result of a pressure difference between a front end and a rear end of the rotor 7, a cooling airflow channel is formed in which cooling air flows through the inside of the rotor 7 from the front end to the rear end.
As a result, heat generated in the stator winding 16 is dissipated from the coil end groups 16f and 16r to the cooling airflows, suppressing temperature increases in the stator 8. Furthermore, heat generated in the rectifier 12 and the regulator 18 is dissipated to a cooling airflow by means of the rectifier heat sink 12a and the regulator heat sink 17, thereby suppressing temperature increases in the rectifier 12 and the regulator 18. In addition, heat generated in the field winding 13 is dissipated to the cooling airflow which flows through the inside of the rotor 7, thereby suppressing temperature increases in the rotor 7.
In the conventional automotive alternator constructed in this manner, it is important to suppress temperature increases in the stator winding 16, which is the largest heat-generating part. Since the cooling airflow formed by the cooling fans 5 and the rotor 7 flows through the coil end groups 16f and 16r of the stator winding 16 from an inner circumferential side in a radial direction, heat generated in the stator winding 16 is dissipated from the coil end groups 16f and 16r to the cooling airflow, thereby suppressing temperature increases in the stator 8.
Now, in an actual alternator, the ambient temperature under the worst operating conditions is 90xc2x0 C. Furthermore, a varnish which is impregnated into the slots 33 of the stator core 15 and which bonds the stator core 15 and the stator winding 16 has a softening temperature of 230xc2x0 C. Thus, when decreased output caused by decreased field current due to increased ambient temperature is taken into consideration, the temperature of the stator 8 can be prevented from exceeding the softening temperature of the varnish even under the worst operating conditions if temperature increases in the stator core 8 are suppressed to 140xc2x0 C. or less at an ambient temperature of 90xc2x0 C. A temperature increase of 140xc2x0 C. at an ambient temperature of 90xc2x0 C. corresponds to a temperature increase of 170xc2x0 C. at an ambient temperature of 20xc2x0 C.
If the varnish reaches its softening temperature, heat degradation is promoted and bonding between the stator core 15 and the stator winding 16 is loosened. Loosening of bonding between the stator core 15 and the stator winding 16 leads to rubbing between the conductor wires 29 of the stator winding 16 and the stator core 15, damaging the electrically-insulating coating of the conductor wires 29 and causing deterioration in electrical insulation.
The present applicants have focused on ventilation channels passing through gaps between the coil end groups 16f and 16r of the stator winding 16 and end surfaces of the stator core 15, and have found that a ratio (bt/ht) between a width bt and a radial length ht of the tooth portions 31, which define these ventilation channels as shown in FIG. 26, affects the cooling of the stator winding 16.
However, until now, no consideration had been given to the ratio (bt/ht) between the width bt and the radial length ht of the tooth portions 31. The stator core 15 used in the conventional automotive alternator has a ratio bt/ht approximately equal to 0.42 (bt=4.8 mm; ht=11.4 mm), for example. When a saturation temperature of the stator 8 was measured with the automotive alternator generating power at full load under stable output conditions, the value of increase in the saturation temperature from an experimental ambient temperature (20xc2x0 C.) was calculated to be 173xc2x0 C. Consequently, in the conventional automotive alternator one problem has been that, under the worst operating conditions, the temperature of the stator 8 exceeds the softening temperature of the varnish, promoting heat degradation and causing electrical insulation to deteriorate.
The present invention aims to solve the above problems and an object of the present invention is to provide an automotive alternator enabling heat degradation tolerance to be improved and deterioration in electrical insulation to be suppressed by appropriately setting a ratio (bt/ht) between the width bt and the radial length ht of tooth portions such that heat dissipation of a stator winding is improved, thereby keeping the temperature of a stator below a softening temperature of a varnish even under the worst operating conditions.
In order to achieve the above object, according to one aspect of the present invention, there is provided an automotive alternator including:
a shaft rotatably supported by a case;
a rotor fixed to the shaft, the rotor being provided with:
a field winding for generating a magnetic flux on passage of an electric current therethrough; and
a plurality of claw-shaped magnetic poles disposed circumferentially on an outer circumferential side of the field winding, the claw-shaped magnetic poles being magnetized by the magnetic flux generated by the field winding; and
a stator provided with:
a cylindrical stator core supported by the case so as to envelop the rotor, a plurality of slots extending axially being formed in the stator core so as to line up circumferentially; and
a stator winding installed in the stator core,
xe2x80x83wherein the stator core is constructed by laminating a magnetic steel plate, the stator core being provided with:
a cylindrical base portion;
a plurality of tooth portions disposed so as to extend from the base portion toward an axial center; and
the plurality of slots, each of the slots being defined by the base portion and an adjacent pair of the tooth portions;
ventilation channels are formed by a coil end group of the stator winding and the tooth portions of the stator core, a cooling airflow generated by rotation of the rotor flowing through each of the ventilation channels in a radial direction from an inner circumferential side; and
each of the tooth portions ia formed such that a radial length ht and a width bt thereof satisfy an expression 0.15 less than bt/ht less than 0.4.
A cooling fan may be fixed to an axial end surface of the rotor.
Air discharge apertures may be formed in a radial side surface of the case so as to correspond to the ventilation channels.
An entire axial length of a blade of the cooling fan may substantially overlap the coil end group in a radial direction.
The stator winding may be installed in the stator core as a distributed winding.
The stator winding may be constituted by a plurality of winding sub-portions each constructed by installing an electrical conductor so as to alternately occupy an inner layer and an outer layer in a slot depth direction in the slots at a predetermined slot interval.
The slots may be formed at a ratio of two or more per phase per pole.
The ventilation channels may be arranged at a non-uniform pitch.